14.118 µV/m at 30 meters

[am1670acr]

All,

I would like to set up my transmitter (Rangemaster) on a post protruding one foot above the ground with 30 – 80' #16 bare copper wire radials with transmitter ground and radials all connected to an 8' ground rod. I would be transmitting on 1610 kHz with 100mW. Our ground conductivity here is about 25.

According to your calculations, would this setup be in compliance with FCC Part 15.209 rules which allow for a maximum of 14.118 microvolts per meter (µV/m) at 30 meters for transmitters in the AM band?

What would the field strength be?

 

[wkbam1690]

> All,
>
> I would like to set up my transmitter (Rangemaster) on a
> post protruding one foot above the ground with 30 – 80' #16
> bare copper wire radials with transmitter ground and radials
> all connected to an 8' ground rod. I would be transmitting
> on 1610 kHz with 100mW. Our ground conductivity here is
> about 25.
>
> According to your calculations, would this setup be in
> compliance with FCC Part 15.209 rules which allow for a
> maximum of 14.118 microvolts per meter (µV/m) at 30 meters
> for transmitters in the AM band?
>
> What would the field strength be?
>
If you are operating at 100 milliwatts and a 3 meter antenna, then you are governed by Part 15.219, which does not specify a field strength, but transmitter power input and antenna length.

 

[am1670acr]

This is a pertinent question, under the 15.209 rules, what would the Field Strength be?

>> All,
>>
>> I would like to set up my transmitter (Rangemaster) on a
>> post protruding one foot above the ground with 30 – 80'
>> #16 bare copper wire radials with transmitter ground and
>> radials all connected to an 8' ground rod. I would be transmitting
>> on 1610 kHz with 100mW. Our ground conductivity here is
>> about 25.
>>
>> According to your calculations, would this setup be in
>> compliance with FCC Part 15.209 rules which allow for a
>> maximum of 14.118 microvolts per meter (µV/m) at 30 meters
>> for transmitters in the AM band?
>>
>> What would the field strength be?
>>
>
> If you are operating at 100 milliwatts and a 3 meter
> antenna, then you are governed by Part 15.219, which does
> not specify a field strength, but transmitter power input
> and antenna length.
>

 

[radio8z]

> All,
>
> I would like to set up my transmitter (Rangemaster) on a
> post protruding one foot above the ground with 30 – 80' #16
> bare copper wire radials with transmitter ground and radials
> all connected to an 8' ground rod. I would be transmitting
> on 1610 kHz with 100mW. Our ground conductivity here is
> about 25.
>
> According to your calculations, would this setup be in
> compliance with FCC Part 15.209 rules which allow for a
> maximum of 14.118 microvolts per meter (µV/m) at 30 meters
> for transmitters in the AM band?
>
> What would the field strength be?
>
Hi,

You are incompliance if you meet the criteria in 15.209 OR 15.219. Part 15.215 allows this.

 

[rfry]

> I would like to set up my transmitter (Rangemaster) on a
> post protruding one foot above the ground with 30 – 80' #16
> bare copper wire radials with transmitter ground and radials
> all connected to an 8' ground rod. I would be transmitting
> on 1610 kHz with 100mW. Our ground conductivity here is
> about 25.
>
> According to your calculations, would this setup be in
> compliance with FCC Part 15.209 rules which allow for a
> maximum of 14.118 microvolts per meter (µV/m) at 30 meters
> for transmitters in the AM band?
>
> What would the field strength be?
__________

I was waiting for anyone else to respond with numbers, but here is one approach.

A general estimate can be made by assuming that, for the same applied power, your antenna system might produce a groundwave field strength about 22 dB below that of a 1/4-wave vertical having a ground system of 120 radials, each 0.4 wavelengths long, this 1/4-wave system being essentially a perfect radiator.

Using the inverse field equation, that 1/4-wave vertical with 100 mW of applied power generates a groundwave field of about 73,874 µV/m at a horizontal distance of 30 meters. This value is mathematically correlated to the 137.6 mV/m groundwave field generated at 1 mile by 1 kW of radiated power, by a perfect 1/4-wave vertical -- which is an FCC standard benchmark.

The groundwave field strength at 30 meters corresponding to the 22 dB reduction estimated for your antenna system is 5,864 µV/m.

So this prompts a question. Why does the FCC refer to 15.209 in citations for excess Part 15 AM field strength when the field strength that might be produced under another one of their Rules by a tx with 100 mW of input power using a ground-mounted 3-meter vertical antenna can exceed the 15.209 value by a factor of several hundred?

//

 

[KeithHamilton]

Here my 2 cents,
You can choose to operate under 15.209, if you do you will need expensive equipment to know if you are at the correct level

Frequency Field Strength Measurement Distance
(MHz) (microvolts/meter) (meters)
_______________________________________________________________
0.490 - 1.705 24000/F(kHz) 30

OR

you can choose to operate under 15.219 which allows 100mw and a 3 meter antenna and is the system that the Rangemaster was certified with.

Yes the Rangemaster will exceed the 15.209 numbers operating under 15.219, the certification application to the FCC clearly showed that. The test lab (cost dearly) did extensive field testing at multiple frequencies. I'm sure this would be true of all Part 15 transmitters. (I think, haven't tried it)

 

[radio8z]

Keith could you give us some numbers?

> Here my 2 cents,
> You can choose to operate under 15.209, if you do you will
> need expensive equipment to know if you are at the correct
> level
>
> Frequency Field Strength Measurement Distance
> (MHz) (microvolts/meter) (meters)
___> ____________________________________________________________
>
> 0.490 - 1.705 24000/F(kHz) 30
>
> OR
>
> you can choose to operate under 15.219 which allows 100mw
> and a 3 meter antenna and is the system that the Rangemaster
> was certified with.
>
> Yes the Rangemaster will exceed the 15.209 numbers operating
> under 15.219, the certification application to the FCC
> clearly showed that. The test lab (cost dearly) did
> extensive field testing at multiple frequencies. I'm sure
> this would be true of all Part 15 transmitters. (I think,
> haven't tried it)
>

Hi Keith,

Are you able to give some numbers from these tests? It would be really interesting to see how they compare to rfry's estimates, and the 209 numbers.

Neil

 

[JasonW]

The FCC is pragmatic

Keith, having federal government employees in my family, I know that agencies often adopt rules that are designed to make their job of enforcement easier (they have a built-in "fudge factor," so to speak).

The FCC Part 15.219 rules (which can be used instead of the Part 15.209 rules if desired) are a perfect example of this. The FCC is willing to accept that a transmitter/antenna/ground system built to comply with Part 15.219 will somewhat exceed the field strength specified in Part 15.209 because enforcement is much easier.

An FCC field agent needs no test equipment to determine compliance with Part 15.219, as it can be done by visual inspection of the installation. The transmitter should have an FCC certification (or type acceptance) label, the antenna should be the one specified by the transmitter manufacturer (a 2.59 meter [102"] CB whip used for outdoor installations with all currently-available transmitters and ATUs) or the manufacturer-provided 3 meter wire antenna for indoor use (Talking House and TalkingSign).

Determination of the compliance of the ground lead and ground portion is up to the field agent's discretion. A 1/4 wavelength or comparable-length ground wire connected to nothing at the far end (an intentionally radiating counterpoise) is prohibited, but a transmitter grounded to a metal mast or tower, or with a ground wire running straight down to a ground rod (so long as the ground wire is a small fraction of a wavelength long) is permitted because of the requirement for DC safety grounding of the transmitter. Some field agents want a Metal Oxide Varistor (MOV) lightning arrestor at the transmitter or ATU ground terminal (so that there is no DC continuity with the ground wire, mast, or tower except during a direct or nearby lightning strike), while others don't require them. -- JasonW

> Here my 2 cents,
> You can choose to operate under 15.209, if you do you will
> need expensive equipment to know if you are at the correct
> level
>
> Frequency Field Strength Measurement Distance
> (MHz) (microvolts/meter) (meters)
___> ____________________________________________________________
>
> 0.490 - 1.705 24000/F(kHz) 30
>
> OR
>
> you can choose to operate under 15.219 which allows 100mw
> and a 3 meter antenna and is the system that the Rangemaster
> was certified with.
>
> Yes the Rangemaster will exceed the 15.209 numbers operating
> under 15.219, the certification application to the FCC
> clearly showed that. The test lab (cost dearly) did
> extensive field testing at multiple frequencies. I'm sure
> this would be true of all Part 15 transmitters. (I think,
> haven't tried it)
>

 

[radio8z]

Re: The FCC is pragmatic

> Keith, having federal government employees in my family, I
> know that agencies often adopt rules that are designed to
> make their job of enforcement easier (they have a built-in
> "fudge factor," so to speak).
>
> The FCC Part 15.219 rules (which can be used instead of the
> Part 15.209 rules if desired) are a perfect example of this.
> The FCC is willing to accept that a
> transmitter/antenna/ground system built to comply with Part
> 15.219 will somewhat exceed the field strength specified in
> Part 15.209 because enforcement is much easier.
>
> An FCC field agent needs no test equipment to determine
> compliance with Part 15.219, as it can be done by visual
> inspection of the installation. The transmitter should have
> an FCC certification (or type acceptance) label, the antenna
> should be the one specified by the transmitter manufacturer
> (a 2.59 meter [102"] CB whip used for outdoor installations
> with all currently-available transmitters and ATUs) or the
> manufacturer-provided 3 meter wire antenna for indoor use
> (Talking House and TalkingSign).
>
> Determination of the compliance of the ground lead and
> ground portion is up to the field agent's discretion. A 1/4
> wavelength or comparable-length ground wire connected to
> nothing at the far end (an intentionally radiating
> counterpoise) is prohibited, but a transmitter grounded to a
> metal mast or tower, or with a ground wire running straight
> down to a ground rod (so long as the ground wire is a small
> fraction of a wavelength long) is permitted because of the
> requirement for DC safety grounding of the transmitter.
> Some field agents want a Metal Oxide Varistor (MOV)
> lightning arrestor at the transmitter or ATU ground terminal
> (so that there is no DC continuity with the ground wire,
> mast, or tower except during a direct or nearby lightning
> strike), while others don't require them. -- JasonW
>

Hi Jason and others,

Your post is interesting, but may I suggest, for now, that we leave the FCC and the rules out of the discussion?

I would like to see some real measured field strength data for part 15 AM systems and I would like to compare these to the predictions made by engineering calculations. The reason for this is so we can gain confidence in the models and calculations and predict results before spending the time and trouble building things. I have seen on this board calculated field strengths but I have not seen measured field strengths. If I missed it, please advise.

I cannot make accurate field strength measurements, but if someone on this board can or did, it would be beneficial to share this with us.

You may quote me: "With the data you do not have, I will establish what you do not know.".

Neil

 

[radio8z]

Jason, please elaborate on MOVs

Hi Jason,

In your post in this thread you wrote:

"Some field agents want a Metal Oxide Varistor (MOV) lightning arrestor at the transmitter or ATU ground terminal (so that there is no DC continuity with the ground wire, mast, or tower except during a direct or nearby lightning strike), while others don't require them."

They do? I didn't know the FCC regulates this. Please elaborate.

Neil

 

[JasonW]

Re: Jason, please elaborate on MOVs

Over the years, some Part 15 AM station operators have been told this by FCC field agents after being inspected. It's not in the regulations--some field agents have specified these in individual cases.

I think they're cases of disinterested or sympathetic agents (often "sicced" onto Part 15 AM stations by nearby licensed stations) trying to simultaneously satisfy the complainers, not shut down the Part 15 stations, and cover themselves.

A ground lead connected to a Part 15 AM transmitter through a MOV arrestor is open at DC (except for a direct or nearby lightning strike), but the arrestor will conduct the RF (by capacitive coupling) with small losses. -- Jason

> Hi Jason,
>
> In your post in this thread you wrote:
>
> "Some field agents want a Metal Oxide Varistor (MOV)
> lightning arrestor at the transmitter or ATU ground terminal
> (so that there is no DC continuity with the ground wire,
> mast, or tower except during a direct or nearby lightning
> strike), while others don't require them."
>
> They do? I didn't know the FCC regulates this. Please
> elaborate.
>
> Neil
>

 

[rfry]

Re: Jason, please elaborate on MOVs

> A ground lead connected to a Part 15 AM transmitter through
> a MOV arrestor is open at DC (except for a direct or nearby
> lightning strike), but the arrestor will conduct the RF (by
> capacitive coupling) with small losses. -- Jason
______________

But wouldn't such an MOV also need to pass the RF current that needs to flow between the tx chassis and earth ground? Otherwise the radiation efficiency would be even worse for an elevated Part 15 system with a long path to earth ground than if it was mounted directly on the ground, without the MOV.

If the MOV doesn't make much difference to the radiation efficiency of the elevated antenna, then it really isn't doing anything to isolate the Part 15 system from the gain increase that results from using the long, radiating path to earth ground.

//

 

[JasonW]

Re: Jason, please elaborate on MOVs

A Metal Oxide Varistor arrestor is an open circuit to DC except under the *very* high voltages that are present with lightning discharges.

Under normal conditions it is an open circuit to DC but a closed circuit to AC (it acts as a capacitor). Since RF is AC, it will flow through an MOV arrestor.

Just as an ordinary capacitor in series with an antenna or counterpoise wire electrically shortens it, an MOV arrestor does electrically shorten the ground wire/ground combination somewhat. It still provides considerably higher performance than a ground-mounted unit, it preserves DC grounding for lightning protection, and it satisfies the FCC's requirements. -- JasonW

> But wouldn't such an MOV also need to pass the RF current
> that needs to flow between the tx chassis and earth ground?
> Otherwise the radiation efficiency would be even worse for
> an elevated Part 15 system with a long path to earth ground
> than if it was mounted directly on the ground, without the
> MOV.
>
> If the MOV doesn't make much difference to the radiation
> efficiency of the elevated antenna, then it really isn't
> doing anything to isolate the Part 15 system from the gain
> increase that results from using the long, radiating path to
> earth ground.

 

[rfry]

Re: Jason, please elaborate on MOVs

> Under normal conditions it is an open circuit to DC but a
> closed circuit to AC (it acts as a capacitor). Since RF is
> AC, it will flow through an MOV arrestor.
>
> Just as an ordinary capacitor in series with an antenna or
> counterpoise wire electrically shortens it, an MOV arrestor
> does electrically shorten the ground wire/ground combination
> somewhat. It still provides considerably higher performance
> than a ground-mounted unit, it preserves DC grounding for
> lightning protection, and it satisfies the FCC's
> requirements. -- JasonW
____________

Doesn't your statement that the MOV idea satisfies the FCC even though its use with an elevated antenna mean, in effect, that the FCC doesn't care how long the active radiator is for Part 15 AM? That doesn't seem really likely.

Can you (or anyone) produce any documentary evidence from the FCC showing that they have no issues with an elevated Part 15 AM antenna having a long, conducting path to earth ground when/if an MOV is installed in series with it?

//

 

[KeithHamilton]

Re: Keith could you give us some numbers?

Have been out of town, I do check in once in a while, very interesting, I have data from a field test, RangeMaster 5 foot high 6500 uv/m compared to fry's 5864 uv calculation.

 

[rfry]

Re: Keith could you give us some numbers?

> Have been out of town, I do check in once in a while, very
> interesting, I have data from a field test, RangeMaster 5
> foot high 6500 uv/m compared to fry's 5864 uv calculation.
__________

My calculation was for a freq of 1.7 MHz applied to a matched 3-m transmit antenna at ground level, and a total of 20 ohms loss in the coil and ground system. Note that ground system loss is not the DC loss of the wire(s) leading to earth ground, it is the r-f loss of those wires, plus the r-f losses seen by the earth currents induced by antenna radiation returning to that earth ground connection.

Your test antenna was higher (had a longer ground conductor, which adds to system radiation), and the freq and system losses weren't stated. So it is impossible to know whether your measurement is at, above, or below its expected value.

//

 

[MonsterFM-KBKH]

Re: Keith could you give us some numbers?

> Your test antenna was higher (had a longer ground conductor,
> which adds to system radiation), and the freq and system
> losses weren't stated. So it is impossible to know whether
> your measurement is at, above, or below its expected value.


I would dearly love to do some "hands on" study of this matter and I have the FIM-41 with which to do it. Unfortunately, I only have two LPB transmitters (60 watt and 30 watt) with which to measure. No true "Part-15" equipment on hand...

Richard, what would you think would be the results of runnig just the oscillator/modulator of an LPB? Aren't they supposed to be 100mw. output? It certainly would be if the DC input was carefully monitored but, would this meet with everyone's expectations? Hey! I'm even willing to apply for an "experimental" license to be able to test these things at both above as well as below Part-15 levels and report back. (Even though THAT option would take some time, obviously, I'm in a secluded enough area that the denial of the request would be highly unlikely and we might all learn something from it.)

Let me say this first and foremost: I am NOT willing to do anything that could jeopardize my commercial FM station at this location! Anything I do *WILL* be authorized or Part-15 ***TO THE LETTER OF THE RULES AND REGULATIONS***!!! But, I will also report back honestly, if I do any form of experimenting with this.
<P ID="signature">______________
Terry Keith Hammond

Message Boards: http://www.monsterfm.com/cgi-bin/ultimatebb.cgi</P>

 

[rfry]

Re: Keith could you give us some numbers?

> Richard, what would you think would be the results of
> runnig just the oscillator/modulator of an LPB? Aren't they
> supposed to be 100mw. output? It certainly would be if the
> DC input was carefully monitored but, would this meet with
> everyone's expectations?

The most representative test would be to use a Part 15 AM tx, as it should have an output network designed to match into the range of base impedances common for a 3-meter, medium-wave vertical -- although that may take a loading coil external to the tx.

Probably a Part 15 AM tx manufacturer or rep is in the best position to do this, as s/he will best be able to optimize the match between the tx and the antenna. Of course the loss to r-f earth currents returning via the ground system will play a big factor, and the ground system should be defined (maybe in several forms such as very poor-average-very good). Ground returns via the cable shield on the program audio line, and by the DC/AC power supplies also need to be considered. Sometimes r-f can be carried quite a distance along power lines, if the Part 15 tx power supply is not decoupled from them well enough.

Really, if Keith Hamilton could give us sufficient details about the hardware and operating configuration used in the FCC work that he referenced here, and how its field strength was measured, that would give us a good data point.

//

 

[radio8z]

Thanks Keith

> Have been out of town, I do check in once in a while, very
> interesting, I have data from a field test, RangeMaster 5
> foot high 6500 uv/m compared to fry's 5864 uv calculation.
>
Keith,

Thanks for the data. I think rfry's model is pretty close considering the unknowns for the model.

Neil

 

[radio8z]

MOV's in series?

Hi all,

I never heard of MOV's being connected in series with anything. They connect in parallel and are opens until a certain voltage (typically 200 volts or so) is reached and they conduct shunting and dissipating the energy. They also wear out. That's why I was curious about using them in an antenna ground system. It may make sense to connect one from the transmitter antenna output to ground, but I don't see why to put it in series with the ground.

It just does not make technical sense.

Neil